Remote sensing apparatus and methods

ABSTRACT

A lidar spectroscopic apparatus comprises a laser transmitter and a receiver in which the radiation return is spectranalyzed, a fluorescent target at a considerable distance from said transmitter and receiver, and means for aiming said transmitter and receiver at said fluorescent target. The presence of pollutants between the lidar system and said target is deduced from the attenuation of the radiation return at several wavelengths and/or from the Raman backscatter due to specific air pollutants.

United States Patent 1 Zaromb REMOTE SENSING APPARATUS AND METHODS [76]Inventor: Solomon Zaromb, 376 Monroe St.,

Passaic, NJ. 07055 [22] Filed: Jan. 4, 1971 [21] Appl. No.1 103,507

[52] US. Cl. 356/103, 356/75, 250/71 R [51] Int. Cl. G0ln 21/00, GOlj3/44 [58] Field of Search 356/103, 207, 256, 356/75; 250/71, 218,104

[56] References Cited UNITED STATES PATENTS 3,528,740 9/1970 Gerry et a1356/75 3,566,114 2/1971 Brewer 250/71 R 3,499,159 3/1970 Carrier et a].356/103 3,540,829 11/1970 Collinson et al 356/129 OTHER PUBLICATIONS NewFields For Laser Raman Spectroscopy, Elec- 11 3,768,908 1 Oct. 30, 1973tro-Optical Systems Design Conf., Toman Hirschfeld, 1970.

Primary ExaminerRonald L. Wibert Assistant ExaminerConrad Clark [57]ABSTRACT A lidar spectroscopic apparatus comprises a laser transmitterand a receiver in which the radiation return -is spectranalyzed, afluorescent target at a considerable distance from said transmitter andreceiver, and means for aiming said transmitter andreceiver at saidfluorescent target. The presence of pollutants between the lidar systemand said target is deduced from the attenuation of the radiation returnat several wavelengths and/or from the .Raman backscatter due tospecific air pollutants.

16 Claims, 9 Drawing Figures Patented Oct. 30, 1973 5 Sheets-Sheet 1FIGJ Patented Oct. 30, 1973 5 Sheets-Sheet 2 Patented Oct. 30, 19733,768,908

5 Sheets-Sheet :5

Patented Oct. 30, 1973 v r 3,768,908

5 SheetsSheet 4 Patented Oct. 30, 1973 3,768,908

5 Sheets-Sheet 5 REMOTE SENSING APPARATUS AND METHODS This inventionrelates to methods and apparatus for remote sensing of invisible airpollutants.

It is an object of this invention to provide optical means of detectingsuch commonly encountered air pollutants as sulfur dioxide, nitrogendioxide, and ozone at considerable distances from an observation point.

It is a further object of my invention to provide such remote sensingmeans utilizing a relatively simple system which can be manufactured atrelatively moderate cost.

It is yet another object of my invention to provide a system capable ofdetecting commonly encountered air pollutants in commonly encounteredconcentrations over distances in excess of a mile.

It is still another object of my invention to provide a means ofdifferentiating between the various encountered pollutants, e.g.,between sulfur dioxide and nitrogen dioxide, between chemical pollutantsand particulate matter, and between inorganic and organic or biologicaltypes of aerosols.

My invention consists of an optical radar system capable of yieldingreturn signals of backscattered, reflected or re-emitted radiation at anumber of different wavelengths. By comparing the return signals atseveral ation into components of one half and one quarter of theoriginal wavelength,'i.e., of 0.53-micron and 0.265- micron wavelength.The harmonic generators 7 and 9 consist preferably of properly orientedand optically polished crystals of potassium dihydrogen phosphate and.ammonium dihydrogen phosphate, respectively. Between them is inserted awater cell 11, containing a solution of about 1 weight-percent of coppersulfate in water along a IO-centimeter light-path, which filters outmost of the originall.06-micron radiation. A removable prism 13 may beinserted in the path of the laser beam in order to deflect said beamtowards a second prism 15, which in turn deflects it towards adiverging' lens 17. The latter serves in conjunction with afront-surface mirror 19 to expand the beam cross section while reducingits divergence.

This beam expansion is used only to cut down the divergence of the0.53-micron pulses. The 0.265-micron wavelengths it becomes possible toidentify and estimate the concentrations of the pollutants present alongthe path of the transmitted and of the returned radiation. 7

The transmitted radiation is preferably in the 0.250.3 micron range ofultraviolet wavelengths, such as those obtainable from the fourthharmonic of a neodymium laser or from the second harmonic of anargon-ion laser. The return signals may be provided by the Ramanbackscatter due to atmospheric nitrogen and oxygen and/or any otheratmospheric constituents, present in sufficiently high concentration orelse by a natural or artificial fluorescent background.

My invention is best explained with reference to the drawing in which:

FIG. 1 is a schematic diagram of the optical system used in oneembodiment of my invention;

FIG. 2 is a schematic diagram of the optical system used in an alternateembodiment of my invention;

FIG. 3A and B is a schematic diagrammatic representation of one mannerin which the apparatus of FIG. 1 or FIG. 2 may be applied;

FIG. 4 represents an alternate method in which the same apparatus isused; and

FIG. 5A, B, C, D is a diagrammatic representation of a few alternateways of generating the fluorescent background used in the methodsillustrated by FIGS. 3 and 4. e

The optical system of FIG. 1 consists of a transmitter of intense pulsesof laser radiation, on the right-hand side, and of a receiver, on theleft-hand side, which collects, analyzes, and measures the radiationwhich is returned following transmission of a laser pulse. Eachradiation produced by the fourth harmonic generator 9 has a fairly lowdivergence as is, and is therefore allowed to proceed undeflected, byremoving prism 13, through an ultra-violet-transmitting filter 21 whichabsorbs at the 0.53-micron wavelength. The transmitted beam may thusconsist of either an approximately 0.1- joule pulse of 0.265-micronradiation with prism 13 removed or an approximately l-joule pulse of0.53- micron radiation with prism 13 in place.

The Q-switch 3 is preferably of the rotating prism type. Its rotationabout a vertical axis causes the divergence angles of the laser beams tobe about l0 times wider horizontally than vertically, so that thetransmitted beams have a horizontal divergence angle of about 0.01radian and a vertical divergence angle of about 0.001 radian. Thisapproximately matches the viewing angles of the receiver, as indicatedbelow.

The receiver comprises a Cassegrain-type telescope 23, a selection offilters mounted on independently ro-. tatable filter wheels 25, an Ebertmonochromator 27 and a photomultiplier tube 29. Most of the returnedradiation incident upon the primary mirror 31 of the telescope 23 isdeflected by a secondary mirror 33 through I a cylindrical quartz lens35 towards the slit 37 of monopulse is produced by a neodymium glasslaser (c'omprising a neodymium laser head 1, a Q-switch 3, and aresonant reflector 5) which emits an intense pulse of-1.06- micronwavelength radiation in less than l0" second, and two successive lightfrequency doublers 7 and 9, also called second'and fourth harmonicgenerators, respectively, which convert parts of the transmittedradichromator 27. The latter selects radiation of a desired wavelengthfor measurement purposes. With a lmillimeter slit 'width, themonochromator bandwidth is less than 0.002 micron. To minimize theamount of stray light passing through monochromator 27, the radiationfrom mirror 33 is prefiltered by a suitable selection of light filtersmounted on filter wheels 25. The light transmitted through monochromator27 is finally picked up by photomultiplier 'tube 29, which yields anelectrical current proportional to the number of photons received. v I

The monochromator slits 37 and 39 are each I millimeter by 2 centimeterswith the longer dimension approximately horizontal. The beam of.radiation from mirror 33 is in the form of nearly parallel raysfocusing at a distance of about 300 cm. Hence with lens 35 out of theway the viewing angles of monochromator 27 would be approximately 0.3milliradian vertically and about 7 milliradians horizontally. The axesof curvature of the thick cylindrical lens 35 are approximately parallelto the long-dimension of slits 37 and 39. The beam dian horizontally,thus approximately matching the afore-mentioned divergence angles of thetransmitted laser beams.

With the center axis of the receiver approximately parallel to and atabout centimeters above that of the transmitter, the wedge-shapedvolumes defined by the viewing and divergence angles meet at a distanceof about 100 meters from the lidar system. The latter is thus suitablefor remote sensing measurements at distances in excess of 100 meters.

The afore-described apparatus combines the optical radar or lidar (lightdetection and ranging) technique (also called laser radar) withspectroscopy, and may therefore be briefly referred to as lidarspectroscopic apparatus. I

In an alternate optical system shown in FIG. 2 the transmitter consistsof several gas lasers 41, 42, 43... emitting at several differentwavelengths. These may comprise an argon-ion laser with asecond-harmonic generator inside the laser cavity emitting 0.2573-micron radiation and/or a standard argon-ion laser emitting four or morewavelengths in the 0.45790.5 l45-micron range and/or a krypton-ion laserwith a second harmonic generator inside the laser cavity emitting0.2604-micron radiation.

The outputs of lasers 41, 42, 43,... are deflected by the respectivemirrors 45, 46, 47,... toward mirror 49 which directs the laser beamsinto a Cassegrain telescope 51. This effectively reduces the divergenceof the output beams besides bringing their emission cones into closecoincidence with the field view of the receiver.

The outputs of lasers 41, 42, 43,... may be either continuous (withappropriate light choppers)or else consist of successive pulses of shortduration (about 10 microseconds) at a high repetition rate. Each pulsemay have a relatively low power (about 1 to 10 watts), and the beamdivergence may be about 0.3 milliradian or less.

The return radiation gathered by the primary mirror 53 of telescope 51is concentrated by the secondary mirror 55 into a nearly parallel beamof less than 1 mm cross-section passing through an aperture 57 in mirror49. This aperture also constitutes the entrance slit into polychromator59. The radiation entering through aperture 57 is directed by mirrors 61and 63 onto a diffraction grating 65. The light dispersed by grating 65is directed by mirror 63 into several exit slits 67, 68... correspondingto different measured wavelengths. Any stray light passing through slits67, 68...may be absorbed by selective light filters 69, 70... placed infront of the respective photodetectors 71, 72...

Small portions of the emitted laser outputs backscattered from mirror55, entering the polychromator 59, and picked up by the detectors 71,72,... may serve to trigger the electronic circuitry. as well as tomeasure the emitted laser pulse energies.

The electronic measurements circuitry associated with the opticalsystems of FIGS. 1 and 2 is rather conventional and need not beelaborated upon. It may consist of a simple oscilloscope displaying thesignals from photodetector(s) 29 or 71, 72, preferably with a cameraattachment for recording the oscilloscope traces. Alternately, thephotodetector signals may be processed by a photon counting circuit andthe information derived therefrom can be recorded on tape or on achart-recorder, or fed into a computer, or printed out in tabular form.The various possible ways of electronically processing and recording thephotodetector signals are well known to those skilled in the art, andthe circuit to be used will depend on the types of measurements whichare to be performed.

The applications where the afore-described apparatus may be used includemonitoring the concentrations of specific air pollutants emitted fromknown sources, whether industrial (smokestacks, incinerators) or natural(active volcanoes, burning woods), mapping the distributions of variousair pollutants in and around industrialized areas, or tracking invisibleair pollutant plumes to their sources in cases of sporadic incidents ofpollution of unknown origin. For the latter application the apparatus ofFIG. 1 is mounted on a solid optical bench 73 (cf. FIG. 3A) supported bya beam 75 hinged on a rotatable mount 77 so that bench 73 can beswivelled to any required horizontal or vertical angle. Mount 77 restson a pedestal 79 which is in turn affixed on one or more heavy steelplates 81 shockmounted to the floor of the vehicle (not shown) tominimize the effects of vibrations induced in the vehicle while inmotion. The apparatus is then used to estimate the relative pollutantconcentrations along various directions. By following with the vehiclealong the direction which indicates the highestpollutant levels it maybe possible to track a pollutant plume to its source.

The methods of estimating pollutant concentrations with the apparatus ofFIG. 1 comprise both lidar Raman and lidar absorption spectroscopictechniques. The Raman technique makes use of the fact that some of thelight scattered by gaseous molecules differs in frequency from that ofthe emitted light, and the frequency difference, the so-called Ramanshift," is a characteristic property of each molecular species. Thechief limitation of this technique is that the Ramanscattering intensityis usually weak, so that a rather high concentration of a givenatmospheric constituent must be present to be detectible at anyappreciable distance. It is thus possible to measure atmosphericnitrogen, oxygen, carbon dioxide, and water vapor, but the lessconcentarated constituents are usually not easily measured by thismethod. Nevertheless, since most industrial plumes have a rather highwater and carbon dioxide content, these two substances may serve asuseful tracers for the tracking of obnoxious fumes to their sources viathe lidar-Raman technique.

The lidar absorption spectroscopic technique makes use of the fact thatsuch pollutants as ozone and sulfur dioxide have strong light absorptionbands in the 0.2650.29 micron wavelength range. The transmitted fourthharmonic 0.265micron laser pulses yield readily measurable signals atthe 0.276-micron and 0.283- micron wavelengths arising from theRaman-shifted backscatter due to atmospheric oxygen and nitrogen,respectively. The second harmonic 0.5 3-micron pulses similarly yieldRaman-shifted signals at the 0.578- micron and 0.605-micron wavelengths.Since the absorption coefficients of ozone and sulfurdioxide are wellknown at these four wavelengths, and there exists at least one fairlygood empirical equation for estimating the atmospheric light attenuationdue to aerosols and particulate matter at the same wavelengths and undervarious visibility conditions, it becomes possible, in principle, toseparate the relative contributions of ozone, sulfur dioxide, andaerosolsor particulate matter to the over-all light attenuation measuredat the four wavelengths, and hence to estimate the approximate averageconcentrations of ozone and sulfur dioxide along a given light travelpath. It may thus be possible to estimate average ozone concentrationsin excess of 0.005 ppm (part per million) and/or sulfur dioxideconcentrations in excess of 0.05 ppm over a distance range of-2-3kilometers.

The main flaw of this measurement technique is that substances otherthan ozone and sulfur dioxide may also absorb strongly at the0.276-micron and 0.283- micron wavelengths. The presence of suchsubstances might then lead to erroneously high estimates of ozone and/orsulfur dioxide concentrations. Nevertheless, when one is attempting totrack an otherwise invisible plume, it may not matter whether one isdetecting ozone, sulfur dioxide, or another obnoxious pollutant. Ineither case will a higher concentration or air pollutants result instronger attenuation of the return signals at 0.276-micron and0.283-micron. On the other hand, the presence of relatively highconcentrations of water and carbon dioxide in a plume may yieldmeasurable Raman backscatter at 0.293 micron and at 0.274 0.275 micron.Hence a pollutant plume may be spotted by relatively weakened'returnsignals at 0.276 micron and at 0.283 micron and by relatively enhancedsignals at 0.274-0.275 micron and at 0.293 micron as compared with thereturn signals at the same wavelengths from relatively pure air.

The afore-disclosed relatively simple plum-e detection procedure arisesfrom a fortunate combination of both afore-mentioned methods, theIidar-Raman and the absorption spectroscopic techniques. The combineduse of these techniques with the same apparatus is made possible in thiscase by the transmission of intense laser pulses of 0.265-micronwavelength. This falls on the onehand within the wavelength transmissionis where transmissionis quite acceptable through unpolluted air butwhere such major air pollutants as sulfur dioxide and ozone absorbstrongly. Furthermore, both the Raman-scattering intensities and thephotodetector sensitivity increase with decreasing wavelength therebyfacilitating signal detection. The latter is also facilitated bynon-interference from ambient daylight, as wavelengths shorter than 0.3micron are removed from sunlight by the upper atmospheric ozone layer.These wavelengths are also strongly absorbed byordinary glass windowsand also by the-corneal fluid of the eye, which eliminates the majorhazard usually encountered with other laser systems, namely thepossibility of retinal eye damage.

The afore-described measurements can, of course, be also performed withthe apparatus of FIG. 2 in lieu of that of FIG. 1. The 0.2573 micronsecond harmonic wavelength of the argon-ion laser would then serve aboutthe same functions as the 0.265 micron fourth harmonic wavelength of theneodymium glass laser. Each of the afore-mentioned Raman-shiftedwavelengths would then be correspondingly shortened by about 0.008micron (the difference between 0.2 65 micron and 0.257 micron).Alternately, the fourth harmonic of the neodymium-doped yttrium aluminumgarnet (or YAG) laser might yield approximately the same wavelengths asthe apparatus of FIG. 1.

The transmitted 0.265 micron or 0.2573 micron pulses may also permitdetection of several air pollutants through measurements of theresonance-Raman backscatter and/or fluorescence return of some of thosesubstances which have absorption bands at or near the transmittedwavelength.

While the exact manner of directly exploiting the two last-mentionedeffects will depend on the results of measurements of theresonance-Raman and/or fluorescence behavior of several pollutantsexcited by either the 0.265 micron or the 0.2573 micron laser radiation,I now proceed to disclose several ways of exploiting the well-knownfluorescence of some dyes and other light absorbing substances for thedetection of air pollutants by the lidar absorption spectroscopictechnique.

The apparatus of either FIG. 1 or FIG. 2 mounted optical opticad bench73 as shown in FIG. 3A may be stationed at a fixed location and besuccessively directed at each of several targets 83, as indicatedschematically in FIG. 3B. An auxiliary viewing telescope (not shownattached to optical bench 73 may be used to facilitate aiming of thelidar system at said targets. Targets 83 preferably comprise fluorescentmaterial disposed on a target surface 85 may be provided by a disposablepaper roll 87 stretched out between two rollers 89 and 91 or by anendless cloth belt 93 stretched between rollers 95 and 97, as indicatedin FIGS. 5A and 58. R0- tation of roller 91 or 95 by a crank 92 or 94results in replacement of a weathered surface by a fresh fluorescentsurface. Such rotation can be effected either manually at appropriateintervals or by a gear drive mechanism powered by an electric motor (notshown) adjusted for a rather slow continuous motion (e.g., 1 meter perday or per week depending on the stability of the fluorescent layer andon ambient weather conditions). Paper roll 87 may be pre-imp regnatedwith fluorescent material and discarded after the entire availablefluorescent surface has deteriorated from prolonged exposure to dust,daylight, and/or other weathering effects, whereas the surface ofendless belt 93 may be continuously renewed by immersion in a bath 99comprising a fluorescent paint 101 protected by a cover 103. Thefluorescent material in targets 83 is selected according to itsfluorescence spectrum. The latter must overlap with the absorptionspectra of the measured air pollutants. For instance, to measure theaverage concentrations of sulfur dioxide, ozone, and nitrogen dioxidebetween the lidar apparatus and targets 83, the fluorescence spectrum oftargets 83 should extend from about 0.28 micron to about 0.4 micron.Such a fluorescence spectrum can be provided by many biologicalmaterials, especially by proteins, and especially by the serum albuminsof most domestic and other animals (horse, beef, sheep, pig, dog, orrabbit). On the other hand, for measurements of nitrogen dioxide alonethe fluorescent spectrum should preferably extend from about 0.35 micronto about 0.5 micron. The latter fluorescence range or major portionsthereof can be readily provided by a number of organic substances suchas sodium salicylate, anthracene, pyrene, 1,2-benzanthracene, 4-methylumbelliferone, sodium 3-ethylaminopyrene-5,8, IO-trisulfonate and manyothers. By comparing the spectra of the radiation returned from the'various observed targets 83 with their known fluorescent specturm, oneobtains the absorption spectra of the atmosphere between the lidarsystem and these targets. From the absorption spectra one can estimatethe concentrations of those air pollutants which absorb at any of thefluorescence wavelengths, and hence one arrives at the distribution ofpollutants around the lidar system.

Each of the targets 83 is protected from direct sunlight by a shield105. With the field of view of the lidar receiver confined to the targetarea, and the latter in shadow, any interference from ambient sunlightbecomes much less troublesome than it is for lidar systems viewing intothe sky. Hence fluorescence measurements at wavelengths in excess of 0.3micron can be performed from the shaded targets even in daytime withoutserious difficulty, especially since the fluorescence return is usuallymuch stronger than the aforediscussed Raman backscatter intensities dueto atmophseric oxygen and nitrogen.

For the technique represented by FIG. 3B, the apparatus of either FIG. 1or of FIG. 2 may be equally suitable. However, the latter can then begreatly simplified, as only one laser wavelength is required to effectthe required fluorescence. It may suffice to use a single argon-ion orkrypton-ion laser 41 (with a second harmonic generator inside the lasercavity) and to dispense with the other two lasers 42, 43,... (andassociated components such, as mirrors 46, 47,...). For measurements ofnitrogen dioxide alone it may even suffice to excite the fluorescence oftargets 83 with a nitrogen laser or with the second harmonic of a rubylaser.

On the other hand, with simple reflective nonfluorescent targets (suchas corner reflectors or mirrors) the apparatus of FIG. 2 would requireseveral lasers in order to perform absorption spectroscopicmeasurements, and such measurements at only a few laser wavelengthscould be easily affected by the presence of interfering substances inthe atmosphere. Thus by providing return signals over a relatively broadspectral region, the fluorescent targets 83 yield more reliablemeasurements with simpler and more economical equipment.

The stationary monitoring arrangement of FIG. 38 may be practical in aheavily industrialized area where a number of possible sources of airpollution may be concentrated within a radius of a few kilometers. Onthe other hand, for more sparsely industrialized areas it may be morepractical to use a mobile monitoring system making rounds among thevarious plants distributed over a larger area. To monitor a singlesmokestack with equipment mounted on a mobile vehicle the apparatus ofFIG. 4 may be appropriate.

FIG. 4 is a schematic representation of a mobile vehicle 107 in whichthe lidar system is mounted. Optical bench 73 carrying the opticalcomponentsis aimed at a portion of a plume 109 emitted from a smokestack111.

The concentrations of the various pollutants in various portions of andaround plume 109 can here again be measured by either the Raman or theabsorption spectroscopic technique. Since the concentration of the majorpollutants at and around the smokestack may be fairly high, their Ramanbackscatter may yield measurable signals if measured at a relativelyshort distance (about 150-200 meters). However, strong absorption in the0.25-0.13 micron range by the various constituents of the plume mayseverely reduce the backseatter signals and thereby introduce anuncertainty in the measurement of any single pollutant species. Thisuncertainty can be reduced by comparing the returns due to eachpollutant with those due to a standard smokestack constituent such ascarbon dioxide. For instance, the rate of carbon dioxide production by apower plant can be estimated from the known rate of fuel consumption.Hence by comparing the Raman returns due to such pollutants as nitricoxide (NO) and sulfur dioxide 8 (S0 with those due to carbon dioxide itbecomes possible to estimate the rates of emission of NO and S0 from agiven stack.

Since the Raman returns due to even relatively high pollutantconcentrations are still rather weak, it may here again be advantageousto utilize a fluorescent target 113 behind the plume and to analyze theabsorption spectra produced by the various portions of said plume and/orby the surrounding atmosphere. Target 113 may consist of a kite orumbrella-- or saucer-shaped opaque sheet 121 of cloth, paper, plastic orother natural or synthetic polymeric material or of thin metal foil(e.g., of aluminum) whose surfaces 123 are covered by a layer offluorescent material yielding a known fluorescence spectrum whenilluminated by the lidar beam (cf. FIGS. 4 and 5C). Target 113 can besupported at the required level above the plume by a small hydrogen orhelium-filled ballon 115 and held in a desired position by strings 117and 119 which can be made to pull at either target 113 or ballon 115. Bymanipulating strings 117 and 119, the inclination of target 113 can beso adjusted that its lower fluorescent surface 123 is facing the lidarsystem while remaining shielded from the sun by the opaque supportingsheet 121. These manipulations may have to be performed continuouslyunder variable wind conditions and/or when several portions of the plumeare to be spectroanalyzed.

While the last-described measurements can be performed with theapparatus of either FIG. 1 or FIG. 2, it is obviously desirable toreduce the duration of these measurements to a minimum. Hence, in theapparatus of FIG. 1, the monochromator 27 with photodetector 29 shouldbe preferably replaced by the polychromator 59 together with themultiple photodetectors 71, 72,... and filters 69, 70,... of. FIG. 2 topermit simultaneous measurements at several wavelengths. Alternately asocalled correlation spectrometer or a similar spectroanalyzer might beused to rapidly estimate the concentrations of specific pollutants basedon their known specific absorption spectra.

Besides extending the applicability of absorption spectroscopicmeasurements, targets 83 or 113 may also permit daytime Ramanspectroscopic measurements simply by serving as shields against a brightsky background. Hence, if the fluorescence spectrum of said targets doesnot extend beyond 0.53 micron, it may be possible to perform improveddirect Raman measurementson various air pollutants using the 0.53-micron second harmonic of the neodymium glass laser of FIG. 1. Such aprocedure may be advantageous in those cases where the opacity of aplume in the 0.250.3 micron range 'may cause excessive attenuation ofthe lidar Raman return in the latter range. The shield provided bytarget 113 also eliminates the hazard of eye damage to aircraftpassengers or crew, even with the 0.5 3-micron laser pulses, providedthat precautions are taken to have the target intercept the entire laserbeam.

Besides their beneficial shielding action, the fluorescent targets 83 or113 in the foregoing illustrative examples also serve the double purposeof providing return signals over a considerably broader spectral rangethan would be obtainable with a few laser wavelengths, and of providingstronger return signals than would be obtainable from the Ramanbackscatter by atmospheric nitrogen and oxygen. The stronger returnsignals arise from the fact that the fluorescence crosssections ofappropriately selected substances are larger by many orders of magnitudethan the Ramanscattering cross-section of air molecules, and also fromthe much higher density of molecules in the fluorescent layers ascompared with the density of air. Of course, retro-reflectors such asthose obtainable with corner prisms can yield even stronger returnsignals at selective laser wavelengths, but such optical components are.more expensive to produce and to maintain than the conditions it maystill be possible to perform improved lidar absorption spectroscopicmeasurements by producing a fluorescent background in the form of afluorescent cloud or plume 125 (FIG. 5D) comprisingeither stronglyfluorescent gases or suspended fluorescent solid particles, aerosols ora similar cluster or agglomeration of small air-borne fluorescentparticles. Among the fluorescent gases may be mentioned sulfur (S oracetaldehyde vapor. Such a cloud can be gener-' ated in several ways,e.g., by exploding one or more shells containing the fluorescentmaterial, said shells being fired from a cannon or similar device, or bydischarging such material from an aircraft.

There will now be obvious to those skilled in the art many modificationsand variations of the afore disclosed apparatus and methods, whichvariations will not depart from the scope of my invention if defined bythe following claims:

I claim:

1. Remote sensing apparatus comprising a laser transmitter and areceiver, said receiver comprising an optical system for focusingradiation incident thereon onto a spectro-analyser, said spectroanalysercomprising means for measuring the radiation received at severalselected wavelengths, said transmitter and receiver being located neareach other and so directed that the laser beam from said transmitterapproximately coincides with the volume viewed by said receiver, afluorescent target at a considerable distance from said transmitter andreceiver, said target having quantitatively established fluorescencecharacteristics, and means for aiming the radiation emitted from saidtransmitter at said fluorescent target.

2. Apparatus as claimed in claim 1 wherein said transmitter transmitsradiation in the 0.25-0.3 micron wavelength range.

3. Apparatus as claimed in claim 2, comprising a neodymium type laserwith second and fourth harmonic generators yielding radiation ofapproximately 0.53 micron and 0.265 micron wavelength, respectively.

4. Apparatus as claimed in claim 1 wherein said target fluoresces in the0.28'0.4 micron range.

5 Apparatus as claimed in claim 1 wherein said target fluoresces in the0.35-05 micron range.

6. Apparatus as claimed in claim 1 wherein the fluorescent material insaid target is supported by a sheetlike polymeric substrate.

7. Apparatus as claimed in claim 1 wherein the fluorescent material insaid target is supported by metal foil.

8. Apparatus as claimed in claim 1 wherein said target is air-borne.

9. Apparatus as claimed in claim 1 comprising means for generating saidtarget in the form of a cluster of numerous small air-borne fluorescentparticles.

10. A method of detecting air pollutants at a distance which comprises:transmitting laser radiation to a fluorescent target, measuring theintensity of the radiation returning from said target at severaldifferent wavelengths, anddeducing the presence of said pollutants fromthe attenuation of the radiation which is emitted by said target.

11. A method as claimed in claim 10 wherein said fluorescent target isair-borne.

12. A method as claimed .in claim 11 wherein said target is lifted by aballon and held down by strings which are manipulated to maintain saidtarget in the desired position.

13. A method as claimed in claim 11 comprising generating a cluster ofair-borne fluorescent particles at a large distance from said lidarsystem.

14. A method of remotely detecting sulfur dioxide and ozone whichcomprises: transmitting laser radiation of at least two distinctwavelengths from a lidar system, at least one of said wavelengths beingin the wavelength range of 0.25 to 0.3 micron, measuring the intensityof the radiation returning to said lidar system at several wavelengthswhich are different than the transmitted wavelengths, some of saiddifferent wavelengths arising from the Raman-scattering of thetransmitted radiation by at least one major atmospheric constituent,especially nitrogen, and comparing the attenuation of said returningradiation at said different wavelengths.

15. A method as claimed in claim 14 wherein the presence of pollutantsis also deduced from an enhancement of said returning radiation atwavelengths corresponding to the Raman-shifted backscatter due to carbondioxide and water.

16. A method as claimed in claim 15 comprising tracking a pollutantplume to its source by measuring the density of the pollutants alongdifferent directions and following along the direction of highestpollutant density.

1. Remote sensing apparatus comprising a laser transmitter and areceiver, said receiver comprising an optical system for focusingradiation incident thereon onto a spectro-analyser, said spectroanalysercomprising means for measuring the radiation received at severalselected wavelengths, said transmitter and receiver being located neareach other and so directed that the laser beam from said transmitterapproximately coincides with the volume viewed by said receiver, afluorescent target at a considerable distance from said transmitter andreceiver, said target having quantitatively established fluorescencecharacteristics, and means for aiming the radiation emitted from saidtransmitter at said fluorescent target.
 2. Apparatus as claimed in claim1 wherein said transmitter transmits radiation in the 0.25- 0.3 micronwavelength range.
 3. Apparatus as claimed in claim 2, comprising aneodymium type laser with second and fourth harmonic generators yieldingradiation of approximately 0.53 micron and 0.265 micron wavelength,respectively.
 4. Apparatus as claimed in claim 1 wherein said targetfluoresces in the 0.28- 0.4 micron range.
 5. Apparatus as claimed inclaim 1 wherein said target fluoresces in the 0.35- 0.5 micron range. 6.Apparatus as claimed in claim 1 wherein the fluorescent material in saidtarget is supported by a sheet-like polymeric substrate.
 7. Apparatus asclaimed in claim 1 wherein the fluorescent material in said target issupported by metal foil.
 8. Apparatus as claimed in claim 1 wherein saidtarget is air-borne.
 9. Apparatus as claimed in claim 1 comprising meansfor generating said target in the form of a cluster of numerous smallair-borne fluorescent particles.
 10. A method of detecting airpollutants at a distance which comprises: transmitting laser radiationto a fluorescent target, measuring the intensity of the radiationreturning from said target at several different wavelengths, anddeducing the presence of said pollutants from the attenuation of theradiation which is emitted by said target.
 11. A method as claimed inclaim 10 wherein said fluorescent target is air-borne.
 12. A method asclaimed in claim 11 wherein said target is lifted by a ballon and helddown by strings which are manipulated to maintain said target in thedesired position.
 13. A method as claimed in claim 11 comprisinggenerating a cluster of air-borne fluorescent particles at a largedistance from said lidar system.
 14. A method of remotely detectingsulfur dioxide and ozone which comprises: transmitting laser radiationof at least two distinct wavelengths from a lidar system, at least oneof said wavelengths being in the wavelength range of 0.25 to 0.3 micron,measuring the intensity of the radiation returning to said lidar systemat several wavelengths which are different than the transmittedwavelengths, some of said different wavelengths arising from theRaman-scattering of the transmitted radiation by at least one majoratmospheric constituent, especially nitrogen, and comparing theattenuation of said returning radiation at said different wavelengths.15. A method as claimed in claim 14 wherein the presence of pollutantsis also deduced from an enhancement of said returning radiation atwavelengths corresponding to the Raman-shifted backscatter due to carbondioxide and water.
 16. A method as claimed in claim 15 comprisingtracking a pollutant plume to its source by measuring the density of thepollutants along different directions and following along the directionof highest pollutant density.